One in seven could still be infectious after five-day Covid isolation

One in seven people who have tested positive for Covid could still be infectious if released from isolation upon receiving a negative lateral flow result after five days, new data suggests.

Across the UK people are now allowed to leave self-isolation on day seven, provided they have had two negative lateral flow tests in the past 24 hours and do not have a fever.

However, the prime minister has indicated he is in favour of further reducing the quarantine time to five days, if backed by scientific evidence. Experts have cautioned there is presently little data to back the move.

According to work previously released by the UK Health Security Agency (UKHSA), the proportion of people estimated to remain infectious five days after the onset of Covid symptoms, or a positive Covid test, is 31%. A small Japanese study has similarly suggested a substantial proportion of those infected with the Omicron variant remain infectious at five days.

New estimates, released by the UKHSA, have suggested the proportion may still remain high even if lateral flow tests are combined with at least five days of isolation.

According to a new report, which is not based on data specific to Omicron, a number of scenarios were modelled including five days of self-isolation followed by five lateral flow tests on days five to nine, with self-isolation ending after a single negative test result.

The team then compared the proportion of infectious people who would be released in this scenario with the situation after 10 full days of self-isolation.

“This increased the proportion of people infectious when released from self-isolation from 5% to 15%,” the team write.

The UKHSA has previously noted that a similar proportion of infectious people – 16% – would be released in a scenario of a seven-day self-isolation period alone, noting this would be “a notably poor solution”.

Modelling from the London School of Hygiene and Tropical Medicine has found “the number of infectious days in the community can be reduced to almost zero” by requiring at least two consecutive days of negative lateral flow tests, “regardless of the number of days’ wait until testing again after initially testing positive”.

However, the authors have cautioned that if most individuals are found to test positive for more than five days, the approach may not address the staffing crisis as desired.

Prof Lawrence Young, a virologist from the University of Warwick, said: “As the UKHSA data attests, reducing the isolation period to five days runs the risk of highly infectious people returning to work or school and could only be introduced with strict enforcement of lateral flow test,” he said. “This raises a concern about compliance – will people use and report their lateral flow test – and requires plentiful supplies of these tests.”

Prof Deborah Dunn-Walters, a professor of immunology at the University of Surrey, said “the public would need to be clearly informed” of the risks involved in leaving isolation early. “[I] can’t imagine anything worse than folks going to visit their vulnerable relatives thinking it is safe when it isn’t,” she said.